(1) Field of the Invention
The present invention relates to the field of lift-generating airfoil surfaces and more particularly to airfoil surfaces forming a rotary wing.
The present invention relates to a blade for a rotary wing aircraft rotor and to a rotor having at least two such blades. The blade is intended more particularly for a main rotor for providing a rotary wing aircraft with lift and possibly also with propulsion.
(2) Description of Related Art
Conventionally, a blade extends longitudinally along its span from a first end for fastening to a rotary hub of a rotor to a second end referred to as its “free” end. Relative to the rotor, it can be understood that the blade extends radially from the first end towards the second end in a spanwise direction. Furthermore, the blade extends transversely from a leading edge towards a trailing edge of the blade along the chord of the blade.
The blade is thus driven in rotation by a rotary hub of the rotor. The axis of rotation of the hub thus corresponds to the axis of rotation of the blade.
The first end is generally referred to below by the term “blade start”, while the free second end is referred to by the term “blade tip”.
In operation, each blade of a rotor is subjected to aerodynamic forces, in particular an aerodynamic lift force during the rotary motion of the rotor, which force serves to provide the aircraft with lift, or indeed with propulsion.
For this purpose, the blade has an airfoil portion situated between the blade start and the blade tip. This airfoil portion is constituted by a succession of airfoil profiles along the span direction, which airfoil profiles are often referred to below for short as “profiles”. Each profile is situated in a transverse plane that is generally perpendicular to the span direction and it defines a section of the blade, being arranged between the start of the airfoil portion and the blade tip. The airfoil portion provides substantially all of the lift of the blade.
The shape of the transition zone between the blade start and the start of the airfoil portion is generally imposed by manufacturing constraints and by structural constraints concerning the blade. This transition zone between the blade start and the start of the airfoil portion may be referred to by the term “blade root” and its aerodynamic performance is much less than the aerodynamic performance of the airfoil portion. The start of the airfoil portion is thus situated between the blade start and the blade tip, in the vicinity of the blade root. This transition zone may nevertheless generate some lift force. In addition, this transition zone, which is situated in the vicinity of the hub of the rotor, nevertheless provides some small contribution to the total lift of the blade, regardless of its aerodynamic shape.
For example, the profiles of the sections of the blade in the airfoil portion are characterized by a thin trailing edge, ideally of zero thickness, whereas the trailing edge in the vicinity of the blade start and of the transition zone between the blade start and the start of the airfoil portion is thick, and possibly rounded.
A rotary wing aircraft presents the advantage of being capable of flying both with high forward speeds during cruising flight and also with forward speeds that are very low, and it is also capable of performing hovering flight. A rotary wing aircraft thus presents the advantage of being able to land on zones of small area and thus, for example, closer to inhabited areas or indeed on landing decks or pads.
Nevertheless, forward flight at high speeds requires the blades to have aerodynamic characteristics that may be different from, or even unfavorable for the characteristics needed for flight at very low forward speeds and for hovering flight.
Likewise, the aerodynamic characteristics of blades also influence the noise generated by blades. Such noise can be problematic during stages of approaching and landing because of the proximity of inhabited areas. Furthermore, strict acoustic certification standards lay down sound levels with which rotary wing aircraft are required to comply.
For a predetermined selection of airfoil profiles, the geometrical characteristics of a blade that have an influence on the aerodynamic performance of the blade during forward flight at high speeds and during hovering flight and also on the acoustic signature of the blade are constituted in particular by the chords of the airfoil profiles of the sections of the blade, by the sweep of the blade, and by the twist of the blade.
It should be recalled that the chord is the distance between the leading edge and the trailing edge of the profile of a blade section. This chord may vary along the span of the blade. The term “taper” is generally used to designate a reduction in chords going along the span of the blade, however this term can also designate an increase in chords along the span of the blade.
Sweep may be defined as the angle formed by the leading edge of the blade with a particular axis of the blade. By convention, in a zone with forward sweep the leading edge forms a sweep angle relative to said blade axis that is positive in the direction of rotation of the rotor, whereas in a zone with backward sweep the leading edge forms a sweep angle relative to said blade axis that is negative. Said blade axis generally coincides with the pitch or feathering axis of the blade.
The twist of a blade consists in varying the setting of the profiles of the sections of the blade along the span of the blade. The term “setting” designates the angle formed between the chord of each profile of the sections of the blade relative to a reference plane of the blade, and this angle is referred to as the “twist” angle. By way of example, the reference plane may be the plane perpendicular to the axis of rotation of the blade and including said blade axis.
The term “twist relationship” designates how the twist angle varies along the span of the blade. In conventional manner, twist is measured as being negative when the leading edge of a section profile of the blade is lower than said reference plane.
Effective solutions are known for independently improving the performance of a blade for high-speed forward flight and the performance of a blade for hovering flight, as well as the acoustic performance of the blade during approach stages.
For example, improving the aerodynamic performance of a blade for hovering flight is characterized by reducing the power drawn by the blade for unchanging rotor lift. This improvement can be obtained by passive changes to the shape of the blade, in particular by increasing its twist.
An appropriate increase in the twist of the blade enables lift to be distributed more uniformly over the entire surface area of the blade and consequently of the rotor, thereby making it possible to reduce the power absorbed by each blade of the rotor in hovering flight. It should be recalled that increasing twist consists in lowering the leading edge relative to said reference plane and doing so more towards the blade tip than towards the blade start because of the variation in the circumferential speed of the air stream as a function of span. The aerodynamic performance of the blade in hovering flight is increased in particular by making the speeds induced along the span of the blade more uniform in this way.
Nevertheless, when the rotary wing aircraft is traveling at high speed, a large amount of blade twist can lead to the blade tip having negative lift, i.e. generating a lift force that is in the same direction as gravity, for a blade that is in an azimuth position known to the person skilled in the art as an “advancing” blade. The aerodynamic performance of the blade is thus degraded in forward flight. Furthermore, the levels of the aerodynamic loads to which the blade is subjected and also the levels of vibration are likewise greatly increased during forward flight.
Adding a dihedral at the blade tip also serves to improve the aerodynamic performance of the blade in hovering flight. A dihedral is formed by a blade surface at the blade tip that slopes upwards or downwards. In hovering flight, the dihedral serves to ensure that the tip vortex generated by any one blade has reduced influence on the following blades of the rotor. Nevertheless, such a dihedral may give rise to a reduction in the aerodynamic performance of the blade in forward flight and also to an increase in vibration.
Furthermore, improving the aerodynamic performance of a blade in forward flight is characterized by reducing the power consumed by each blade of the rotor for given lift and forward speed. This improvement may be obtained by passive modifications to the shape of the blade, and in particular by modifying its chord along the span of the blade and/or by decreasing its twist.
For example, the chord of profiles of sections of the blade increases going from the blade start along the span, and then it decreases before reaching the blade tip. The blade is said to be a “double-tapered” blade. Document EP 0 842 846 describes a double-tapered blade in which the maximum chord is situated at a distance lying in the range 60% to 90% of the total span of the blade from the axis of rotation of the blade.
Nevertheless, the use of a double-tapered blade often gives rise to an increase in noise during approach flight as a result in the increasing intensity of vortices given off by and then impacting against each blade. The use of such a double taper also gives rise to degraded performance in hovering flight compared with a blade having the same twist and the same “blade solidity”, which term designates the ratio of the total area occupied by the blades of the rotor seen from above to the area of the rotor disk, i.e. the area that is swept by a blade of the rotor on rotating through one revolution.
Furthermore, and in compliance with the above, a decrease in the twist of the blade leads to an increase in the aerodynamic angles of attack at the blade tip on the advancing blade side. The angles of attack for a non-twisted blade tip are thus closer to zero on the advancing blade side, thus serving firstly to reduce the negative lift at the blade tip on the advancing blade side and also to reduce local drag, in particular the drag associated with the appearance of shock waves.
In contrast, reducing twist at the end of the blade leads to a reduction in the stall margin of the blade on the retreating blade side. In addition, this reduction in the twist of the blade is unfavorable in hovering flight, as mentioned above.
Documents U.S. Pat. No. 7,252,479 and EP 0 565 413 describe a blade adapted to high-speed forward flight, combining a double-tapered blade with a twist relationship.
Finally, the improvement in acoustic performance of a blade during approach flight may be characterized by reducing the noise that is generated by the interaction between the blade and the air vortex generated by the preceding blades of the rotor. This improvement may be obtained by passive modifications to the shape of the blade, in particular by modifying its sweep along its span.
By way of example, as described in Documents EP 1 557 354, US 2012/0251326, and U.S. Pat. No. 6,116,857, a blade with a first zone that is forwardly-swept and a second zone that is backwardly-swept avoids the leading edge of a blade in these first and second zones being parallel to the lines of vortices given off by the preceding blades. Such a blade can thus limit interactions between the blade and these vortices, e.g. reducing the intensity of impulse noise associated with the interaction between the blade and the vortices, and consequently limiting the appearance of noise.
Furthermore, that blade with two sweeps may also include taper in the backwardly-swept second zone that also serves to reduce the noise level generated in flight. Specifically, for a given profile, the thickness of the blade decreases with shortening chord, thereby decreasing the appearance of so-called “thickness” noise. Likewise, since the area of the blade is reduced as a result of its taper, its lift is also modified, which can reduce the appearance of so-called “load” noise.
It is also possible to act on the aerodynamic load at the blade tip in order to modify the vortices given off in the wake of the blade, and consequently reduce the sound level of the blade. For this purpose, the relationships for variation in the twist and in the chord of the second profiles of the blades are modified. Nevertheless, such variations are incompatible with the above-mentioned optimizations concerning hovering flight or forward flight.
Furthermore, independently of the shape of the blade, it is also possible to modify the speed of rotation of the blade or indeed to adopt specific approach flight paths for the aircraft referred to as “least noise approach flight paths” in order to reduce the noise radiated to the ground by the blades of the aircraft.
Nevertheless, modifying the speed of rotation of the blade makes the work of dynamically balancing the blade more complex. Furthermore, a reduction in the speed of rotation of the blade can give rise in particular to an increase in aerodynamic stalls at the ends of the blade, and consequently to an increase in the dynamic control forces of the blade.
It is also possible to combine applying two sweeps with variations in the chord of the profiles of the sections of the blade with a twist relationship that is adapted either to hovering flight or else to forward flight. Thus, documents EP 1 557 354 and US 2012/0251326 describe blades that are adapted for hovering flight while also enabling a reduction in the noise generated during approach flights. Likewise, document EP 0 842 846 describes a blade that is adapted for forward flight at high speeds and that enables noise to be limited during approach flight.
Nevertheless, the aerodynamic performance of such blades is not optimized for the stage of flight for which the blades are not adapted. Significantly reducing the noise given off by a blade is in any event given precedence, and the aerodynamic performance of the blade may be degraded during certain stages of flight. This degradation is due in particular to a lack of twisting stiffness and/or of bending stiffness of the blade which can then deform under the aerodynamic and inertial forces to which it is subjected.
In contrast, optimizing blade profiles for high-speed forward flight is different and appears to go against optimizing those profiles for hovering flight. Optimizing profiles both for hovering flight and for high-speed forward flight is particularly complex to achieve, since the aerodynamic conditions encountered by the blade are different. Furthermore, during rotation of the rotor, the position of a blade alternates between advancing and retreating relative to the air stream, thereby increasing the differences between the aerodynamic conditions encountered by the blade.
Finally, the document entitled “Multiobjective-multipoint rotor blade optimization in forward flight conditions using surrogate-assisted memetic algorithms”, given to the “European Rotorcraft Forum” at Gallarate (Italy) in September 2011 compares several methods of optimizing a blade in forward flight. The blade may have only a twist relationship, or it may present a combination of relationships for varying chord and sweep, or indeed it may present a combination of relationships for varying twist, chord, and sweep.